Receptor interacting protein-2 (RIP2) kinase, which is also referred to as CARD3, RICK, CARDIAK, or RIPK2, is a TKL family serine/threonine protein kinase involved in innate immune signaling. RIP2 kinase is composed of an N-terminal kinase domain and a C-terminal caspase-recruitment domain (CARD) linked via an intermediate (IM) region ((1998) J. Biol. Chem. 273, 12296-12300; (1998) Current Biology 8, 885-889; and (1998) J. Biol. Chem. 273, 16968-16975). The CARD domain of RIP2 kinase mediates interaction with other CARD-containing proteins, such as NOD1 and NOD2 ((2000) J. Biol. Chem. 275, 27823-27831 and (2001) EMBO reports 2, 736-742). NOD1 and NOD2 are cytoplasmic receptors which play a key role in innate immune surveillance. They recognize both gram positive and gram negative bacterial pathogens and are activated by specific peptidoglycan motifs, diaminopimelic acid (i.e., DAP) and muramyl dipeptide (MDP), respectively ((2007) J Immunol 178, 2380-2386).
Following activation, RIP2 kinase associates with NOD1 or NOD2 and appears to function principally as a molecular scaffold to bring together other kinases (TAK1, IKKα/β/γ) involved in NF-κB and mitogen-activated protein kinase activation ((2006) Nature Reviews Immunology 6, 9-20). RIP2 kinase undergoes a K63-linked polyubiquitination on lysine-209 which facilitates TAK1 recruitment ((2008) EMBO Journal 27, 373-383). This post-translational modification is required for signaling as mutation of this residue prevents NOD1/2 mediated NF-kB activation. RIP2 kinase also undergoes autophosphorylation on serine-176, and possibly other residues ((2006) Cellular Signalling 18, 2223-2229). Studies using kinase dead mutants (K47A) and non-selective small molecule inhibitors have demonstrated that RIP2 kinase activity is important for regulating the stability of RIP2 kinase expression and signaling ((2007) Biochem J 404, 179-190 and (2009) J. Biol. Chem. 284, 19183-19188).
Dysregulation of RIP2-dependent signaling has been linked to autoinflammatory diseases. Gain-of-function mutations in the NACHT-domain of NOD2 cause Blau Syndrome, early-onset sarcoidosis, a pediatric granulomateous disease characterized by uveitis, dermatitis, and arthritis ((2001) Nature Genetics 29, 19-20; (2005) Journal of Rheumatology 32, 373-375; (2005) Current Rheumatology Reports 7, 427-433; (2005) Blood 105, 1195-1197; (2005) European Journal of Human Genetics 13, 742-747; (2006) American Journal of Ophthalmology 142, 1089-1092; (2006) Arthritis & Rheumatism 54, 3337-3344; (2009) Arthritis & Rheumatism 60, 1797-1803; and (2010) Rheumatology 49, 194-196). Mutations in the LRR-domain of NOD2 have been strongly linked to susceptibility to Crohn's Disease ((2002) Am. J. Hum. Genet. 70, 845-857; (2004) European Journal of Human Genetics 12, 206-212; (2008) Mucosal Immunology (2008) 1 (Suppl 1), S5-S9. 1, S5-S9; (2008) Inflammatory Bowel Diseases 14, 295-302; (2008) Experimental Dermatology 17, 1057-1058; (2008) British Medical Bulletin 87, 17-30; (2009) Inflammatory Bowel Diseases 15, 1145-1154 and (2009) Microbes and Infection 11, 912-918). Mutations in NOD1 have been associated with asthma ((2005) Hum. Mol. Genet. 14, 935-941) and early-onset and extraintestinal inflammatory bowel disease ((2005) Hum. Mol. Genet. 14, 1245-1250). Genetic and functional studies have also suggested a role for RIP2-dependent signaling in a variety of other granulomateous disorders, such as sarcoidosis ((2009) Journal of Clinical Immunology 29, 78-89 and (2006) Sarcoidosis Vasculitis and Diffuse Lung Diseases 23, 23-29) and Wegner's Granulomatosis ((2009) Diagnostic Pathology 4, 23).
A potent, selective, small molecule inhibitor of RIP2 kinase activity would block RIP2-dependent pro-inflammatory signaling and thereby provide a therapeutic benefit in autoinflammatory and/or autoimmune diseases characterized by increased and/or dysregulated RIP2 kinase activity.